Reduced fuming fluoropolymer

ABSTRACT

The present invention relates to the reduction of oligomer content of melt-processible fluoropolymer so that the fluoropolymer has at least 25 ppm less oligomer than the as-polymerized fluoropolymer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to melt-processible fluoropolymer that ischaracterized by reduced particulate emissions at elevated temperatures.

2. Description of Related Art

Fluoropolymer can be described as having certain number average orweight average molecular weight, but it is well-known that it is made upof polymer chains of various molecular weights. The smaller polymerchains comprise the lower molecular weight fractions of the polymer andcan be described as oligomers. In melt processing these oligomers havesome volatility and can separate from the mass of molten polymer,forming particulate that can cause polymer fume fever as disclosed inSeidel et al., “Chemical, Physical, and Toxicological Characterizationof Fumes Produced by heating Tetrafluoroethylene Homopolymer and itsCopolymer with Hexafluoropropylene and Perfluoro(propyl vinyl ether)”,Chem. Res. Toxicol. 1991, 4, 229-236.

It is desirable to have fluoropolymer that has reduced particulateemissions at elevated temperatures.

BRIEF SUMMARY OF THE INVENTION

The present invention satisfies this desire by providingmelt-processible fluoropolymer of substantially reduced emission ofparticulates. As determined by the amount of oligomer that can beremoved from fluoropolymer, the trace amounts of oligomer present influoropolymers as reported in Seidel et al. is measurable in parts(weight) per million (ppm). The fluoropolymer of the present inventioncan be characterized by the fluoropolymer having at least about 25 ppmoligomer less than as-polymerized. The amount of oligomer in thefluoropolymer is the result of premature termination of the polymerchain during the polymerization process; this is the as-polymerizedoligomer content of the fluoropolymer.

The fluoropolymer of the present invention can also be characterized bythe process by which it is made, i.e. to reduce the as-polymerizedoligomer content. Thus, the fluoropolymer having a lowered oligomercontent is made by melting the fluoropolymer, creating an infinitesurface of the molten fluoropolymer, contacting this infinite surfacewith gas, and devolatilizing the resultant molten fluoropolymer. Thisembodiment too is preferably carried out to reduce the oligomer contentof the as-polymerized fluoropolymer by at least 25 ppm oligomer.

Since the volatile material that is emitted from the fluoropolymer isparticulate material, which is referred to herein as oligomer, thereduction in oligomer content to obtain the fluoropolymer of the presentinvention can be characterized by reduced emission of particulates,which can be detected by voltage change from an ionizing smoke detector.The fluoropolymer according to this embodiment of the present invention,upon being subjected to the Smoke Detector Test is characterized byexhibiting a voltage change of no more than about 25% of the voltagechange exhibited by the as-polymerized fluoropolymer at the uppercontinuous use temperature of the fluoropolymer. The upper continuoususe temperature for fluoropolymers is established and published by thefluoropolymer manufacturer. The Smoke Detector Test is essentially themeasurement equipment and procedure referred to in Seidel et al. andwill be described in greater detail later herein.

In all of the foregoing embodiments of the present invention, thefluoropolymer is melt processible, since the method of reducing oligomercontent from the as-polymerized amount involves melt processing. Thefluoropolymer is also free of alkali and alkaline earth metal so as toavoid any deleterious effect of such metal on the melt processing of thefluoropolymer.

For melt-processible tetrafluoroethylene/hexafluoropropylene (TFE/HFP)copolymer commonly known as FEP, the reduced emission of particulates ofthe FEP of the present invention is characterized by said FEP exhibitinga voltage change of either no greater than about 0.025 volts at 200° C.or no greater than about 2 volts at 350° C. when subjected to the SmokeDetector Test, said fluoropolymer being free of alkali and alkalineearth metal.

DETAILED DESCRIPTION OF THE INVENTION

The oligomer content of melt-processible fluoropolymers can becharacterized by the reduction of oligomer content under a conditionused in industry most likely to reduce oligomer content. In particular,the melt extrusion of fluoropolymers sometimes includes adevolatilization zone in the extruder just prior to the extrudateexisting the extruder. The high temperature of the fluoropolymer meltwithin the extruder provides the best opportunity for the oligomer tovolatilize and the subsequent devolatilization zone provides the bestopportunity for removal of the oligomer from the molten polymer. It isknown to sparge a bed of fluoropolymer solids after extrusion and atelevated temperatures well below the melting temperature of thefluoropolymer to remove residual gases trapped in the solids, butsparging cannot remove the higher boiling oligomers and the temperatureof sparging must be kept low enough to avoid having the fluoropolymerpellets being sparged stick together, i.e. “block”.

According to the present invention, fluoropolymer is subjected to meltextrusion and devolatization in a special process that increases theopportunity for oligomer removal, and this process is operated in way todistinguish the new, low oligomer content fluoropolymer, from theoligomer content of the same fluoropolymer as-polymerized. The specialprocess involves the melting of the fluoropolymer and creating aninfinite surface for the molten fluoropolymer, similar to the boiling ofa liquid, wherein molecules can depart from the liquid from any and allportions thereof. A critical aspect of this process is that the infinitesurface of molten polymer is contacted with a gas stream. The gas sweepsthe volatile oligomer from the polymer melt. This is apparent from thenext step of subjecting the molten polymer to devolatization, collectingthe oligomer within the vacuum system, and comparing its weight with theweight of fluoropolymer being melt processed. When the gas is omitted,the oligomer removal, as described above, falls to less than 1/10 of theamount removed when the gas is used. The greater amount of oligomer thatis removed when gas is used remains in the fluoropolymer when gas is notused. This amount can be considered to be included in the as-polymerizedamount of oligomer present in the fluoropolymer.

The process just described is not conventional in fluoropolymer meltprocessing, which is generally carried out to minimize the exposure ofthe fluoropolymer to high melt temperature for a period of time thatwould cause the fluoropolymer to degrade and to avoid exposing thefluoropolymer to such high shear that it causes the fluoropolymer todegrade. The exception to this precaution is the intensive extrudershearing of FEP disclosed in U.S. Pat. No. 4,626,587 to eliminatepolymer chain HFP diads and triads, and wherein the degraded polymer issubjected to fluorine treatment after extrusion to eliminate the visualeffects of the degradation. Degradation may be visible by discolorationof the extruded fluoropolymer. Otherwise the degradation becomesapparent from the deterioration of one or more physical properties, suchas flex life, tensile strength, or elongation to break. The specialprocess for making the fluoropolymer of the present invention, however,can be carried out in an extruder that contains a zone in which themolten fluoropolymer is subjected to surface renewal without excessiveshear and within time/temperature condition to avoid degradation, andgas is injected into this zone to contact the constantly regeneratingsurface of molten fluoropolymer to sweep out the volatilizing oligomer.The surface renewal creates the infinite surface, simulating the boilingof a liquid, wherein the amount of surface is not measurable, but itsinfinite nature is indicated by the ability of the gas to sweep oligomerfrom the melt, revealing the extent to which interior portions of themolten resin are brought to the gas/molten polymer interface. The extentof surface renewal is not measured, but its existence is revealed by theremoval of oligomer from the fluoropolymer. The molten fluoropolymer isthen advanced to a devolatilizing zone, where the oligomer is removedfrom the molten polymer. The surface renewal zone and thedevolatilization zone are separated by a plug of molten fluoropolymercreated by the lesser or reverse pitch of the extrusion screw ascompared to the pitch of the screw elements causing the moltenfluoropolymer to advance up to the devolatilization zone. This enablesthe gas to intimately contact the molten fluoropolymer in the surfacerenewal zone without being prematurely removed in the devolatilizationzone. The vacuum applied in the devolatilization zone is small so as toavoid the premature removal of the gas from the surface renewal zone.Notwithstanding the presence of the plug of molten fluoropolymerseparating the surface regeneration zone from the devolatilization zone,the oligomer volatilizing and being swept out of the fluoropolymer inthe surface renewal zone is removed in the devolatilization zone. Afterdevolatilization, the fluoropolymer is cooled.

The surface renewal of the molten polymer occurring in the presence ofgas contact creates the infinite surface of the molten fluoropolymer,enabling the oligomer to be removed from the fluoropolymer. This removaloccurs in a short residence time in the surface renewal zone, usuallyless than about 60 sec.

When this process is practiced on fluoropolymer, the fluoropolymer isbeing subjected to extruder conditions most favorable to removingoligomer from the fluoropolymer, provided that devolatization is alsopracticed in the extruder. When no gas is introduced into the surfacerenewal zone and the molten fluoropolymer is subjected todevolatilization, the amount of oligomer collected from devolatilizationcorresponds to 2 ppm. When the same fluoropolymer is treated in the sameway, except that an inert gas such as N₂ is introduced into the surfacerenewal zone, the amount of oligomer collected is more than 10× theamount collected when no gas is used. At least 25 ppm of oligomer isremoved. When a reactive gas, such as fluorine, is introduced into thesurface renewal zone, the amount of oligomer collected is more than 100×the amount collected when no gas is used. At least 200 ppm of oligomeris removed from the fluoropolymer. Examples with this information anddetails about the extrusion equipment used to obtain this improvementare presented later herein.

While the best result is obtained when a reactive gas such as fluorineis used, the improvement by more than a factor of 10 obtained when anon-reactive gas is used is a valuable contribution. Moreover, when thefluoropolymer contains fluorine-reactive units in the polymer chain suchas ethylene units in tetrafluoroethylene/ethylene copolymer, then only anon-reactive gas should be used.

The fluoropolymer can also be subjected to the Smoke Detector Test,which is essentially the measurement equipment and procedure referred toin Seidel et al. and will be described in greater detail later herein,to measure the reduced particulates of the fluoropolymer of the presentinvention, as the indicator of its reduced oligomer content as comparedto the as-polymerized fluoropolymer.

The special process described above is applicable to fluoropolymers ingeneral, with temperature, tolerable shear, and residence timeconditions dependent on the particular fluoropolymer. This together withthe extruder components (elements) to obtain the condition of infinitesurface in the zone of injected gas are selected to achieve the removalof oligomer without degrading the fluoropolymer. The extruder elementsare described in the Examples. They are selected in accordance with therequirements of the fluoropolymer to obtain the infinite surfacerequired without adversely affecting the fluoropolymer. The Examplesprovide a selection and arrangement of elements, in accordance with theknowledge of the melt processing characteristics of the fluoropolymerbeing melt processed and the need for obtaining the infinite surface toprovide for optimum oligomer removal.

Examples of melt processible fluoropolymers having reduced oligomercontent are copolymers of tetrafluoroethylene (TFE), with one or more ofcomonomers in sufficient amount to render the copolymer meltprocessible. The comonomers can be perfluorinated or other monovalentatoms, such as hydrogen and chlorine, in addition to fluorine can besubstituted on the carbon atom chain as can pendant groups on etherlinkages attached to the carbon atom chain, the fluoropolymernevertheless containing at least about 35 wt % fluorine. By“melt-processible” it is meant that the fluoropolymer flows when heated,as distinguished from polytetrafluoroethylene, which has such a highmelt viscosity that it does not flow when heated. Themelt-processibility of the fluoropolymer also means that it can bemelt-fabricated by such processes as extrusion and injection moldinginto such final articles as films, fibers, tubes, wire coatings and thelike. The removal of oligomer can be part of the melt fabricationprocess to obtain the desired final article or can be separate frommelt-fabrication, to first form pellets that are subsequently used inmelt-fabrication of the desired final article.

Melt processibility generally requires that the melt viscosity be nomore than about 10⁶ Pa·s. Preferably it is in the range of about 10² to10⁶ Pa·s, and most preferably about 10⁴ to 10⁶ Pa·s. Melt viscosities ofthe melt-processible perfluoropolymers are measured according to ASTM D1238-52T, modified as follows: The cylinder, orifice and piston tip aremade of a corrosion-resistant alloy, Haynes Stellite 19, made by HaynesStellite Co. The 5.0 g sample is charged to the 9.53 mm (0.375 inch)inside diameter cylinder, which is maintained at 372° C.±1° C., such asdisclosed in ASTM D 2116 and ASTM D 3307 for perfluorinated polymers.Five minutes after the sample is charged to the cylinder, it is extrudedthrough a 2.10 mm (0.0825 inch) diameter, 8.00 mm (0.315 inch) longsquare-edge orifice under a load (piston plus weight) of 5000 grams.This corresponds to a shear stress of 44.8 kPa (6.5 pounds per squareinch). The melt viscosity in Pa·s is calculated as 53170 divided by theobserved extrusion rate in grams per 10 minutes. The melt viscosity offluoropolymers containing hydrocarbon groups in the polymer chain can bedetermined in accordance with ASTM procedures for these particularpolymers, such as ASTM D 3159 and ASTM D 5575.

One example of fluoropolymer is the copolymer of TFE withhexafluoropropylene (HFP), commonly known as FEP. Additionalcopolymerized monomer may be present in the FEP such as perfluoro(ethylor propyl vinyl ether). Ethylene may also be copolymerized with the TFEand HFP to form EFEP. Another example of fluoropolymer is the copolymerof TFE with perfluoro(alkyl vinyl ether) (PAVE), and perfluorodimethyldioxole (PDD). TFE/PAVE copolymers are commonly known as PFA, which caninclude MFA. PAVE include perfluoro(alkyl vinyl ether), wherein thealkyl group contains from 1-8 carbon atoms, preferably 1 to 3 carbonatoms, such as perfluoro(propyl vinyl ether) (PPVE), perfluoro(ethylvinyl ether) (PEVE), and perfluoro(methyl vinyl ether) (PMVE) ormixtures thereof. The copolymer of TFE/PMVE and PPVE is commonly knownas MFA. PFA is commonly used in extrusion and injection molding to makeprocessing equipment for use in semiconductor manufacture, whereinextreme purity and chemical inertness of the processing equipment isessential. Much technology has been developed to enhance the chemicalinertness of the PFA and to provide smooth surfaces of thePFA-fabricated equipment. The emission of particulates, even in minuteamounts, during semiconductor manufacture, can contaminate thesemiconductor, causing it to be discarded.

Still another example of fluoropolymer is that containing hydrocarbongroups in the polymer chain are copolymers of tetrafluoroethylene orchlorotrifluoroethylene with ethylene, known as ETFE and ECTFE,respectively and copolymers oftetrafluoroethylene/hexafluoropropylene/vinylidene fluoride, known asTHV.

The fluoropolymers may be crystalline or amorphous. By crystalline ismeant that the polymers have some crystallinity and are characterized bya detectable melting point measured according to ASTM D 3418, and amelting endotherm of at least about 3 J/g. Melt-processible polymersthat are not crystalline according to the preceding definition areamorphous. Amorphous polymers include elastomers, which aredistinguished by having a glass transition temperature of less thanabout 20° C. as measured according to ASTM D 3418.

In addition to having reduced oligomer content, the fluoropolymer of thepresent invention should also be free of other possible sources ofcontamination. One of the most common contaminates that might be presentin the copolymer is alkali or alkaline earth metal (ion) such as arisefrom the particular initiator used to carry out the polymerization thatforms the fluoropolymer. The work up of the fluoropolymer from thepolymerization medium to prepare it for feeding to the melt processingequipment includes steps to remove such metal from the polymer, to avoidany adverse effect therefrom in melt processing. In accordance with thepresent invention, no alkali or alkaline earth metal is added to thepolymerization system, whereby the fluoropolymer is free of such metal.Instead, a polymerization initiator that is free of such metal, such asammonium persulfate is used. By substantially free of alkali andalkaline earth metal is meant that the fluoropolymer contains no morethan 10 ppm, preferably no more than 5 ppm of alkali and alkaline earthmetal.

EXAMPLES

All of the melt processing in the Examples is carried out with a 57 mmtwin-screw extruder, equipped with an injection probe, which is a rodhaving a longitudinal bore opening flush with the interior surface ofthe extruder barrel in the surface renewal zone, and a vacuum port inthe devolatilization zone. The twin screw extruder feeds the moltenfluoropolymer into a 120 mm single-screw extruder, which is equippedwith a die. The twin-screw extruder serves as a resin melter andoligomer remover and the single-screw extruder serves as a melt pump togenerate the pressure necessary to move the resin through the screenpack and die. Polymer exiting the die is cut and cooled.

The twin-screw extrusion equipment described above is a Kombiplast®extruder from the Coperion Corporation. Corrosion-resistant materialsare used for those parts that come into contact with the polymer melt.The twin-screw extruder has two corotating screws disposed side by side.The screw configurations are designed with an intermeshing profile andclose clearances, causing them to be self-wiping. The screwconfigurations include kneading blocks, mixing elements, and conveyingscrew bushings. The first 15 Length/Diameter units (L/D, L being theinterior length of the extruder barrel D being the diameter of thebushings) of the extruder is the melting zone. This contains thefeeding, solids conveying, and kneading block sections. The kneadingblock sections provide high shear and insure proper melting of thepolymer. The melting section ends with a left-handed bushing (rearwardpumping) that forms a melt seal and insures complete filling of thefinal kneading blocks. The melt seal forms the entry into the surfacerenewal zone.

The next 19 L/D contain the extruder screw elements that create theinfinite surface of the molten fluoropolymer and convey the moltenfluoropolymer towards the devolatilization zone. The extruder elementsinclude mixing elements that accomplish the surface renewal at low shearas the molten fluoropolymer passes through the surface renewal zone. Theelements making up the surface renewal zone contain one 80 mm undercutconveying bushing (also known as a SK bushing where 80 mm is both thelength of the element and the pitch of the helical flight in onerevolution of the element), one 40 mm conveying bushing that transitionsfrom undercut to standard (also known as a SK—N bushing where 40 mm isthe length of the element and the pitch of the helical flight in onerevolution of the element is 80 mm), two 40 mm conveying bushings (40 mmis both the length of the element and the pitch of the helical flight inone revolution of the element), two 30 mm ZME elements (where 30 mm isthe length of the element, and the pitch of the helical flight in onerevolution of the element is 15 mm), one 40 mm conveying bushing, threeSME elements (40 mm is both the length of the element and the pitch ofthe helical flight in one revolution of the element), two 40 mmconveying bushings, four TME elements (20 mm is the length of theelement and there is no pitch), two 40 mm conveying bushings, three TMEelements, two 40 mm conveying bushings, two ZME elements, two 40 mmconveying bushing, one 30 mm conveying bushing (30 mm is both the lengthof the element and the pitch of the helical flight in one revolution ofthe element) and a 20 mm left-handed bushing (where 20 mm is the lengthof the element and the pitch of the helical flight in one revolution ofthe element is 40 mm) to provide a melt seal with respect to thedevolatilization zone.

The ZME elements are shown as multiple elements in FIG. 4 of U.S. Pat.No. 5,318,358, and as shown in FIG. 1 of the patent are reverse pumpingwith respect to the direction of advancement of the molten fluoropolymerthrough the extruder. These elements also have notches in the peripheryof the screw flight to enable the molten fluoropolymer to be broken intosmall streams traveling in the direction of advancement of thefluoropolymer. The TME elements resemble the ZME elements except thatthey are neutral with respect to pumping action, i.e. they resemble agear. The SME elements resemble the ZME elements except that they areforward pumping. Each ZME, TME, and SME element has at least 10 notchesin the periphery of its respective flights. All these elements arebilobal (two flights per element), except for the ZME which has only onelobe (one flight) and the TME element which is a cylindrical notcheddisc.

A 1 mm thick spacer ring is present between conveying bushings and ZMEelements. The elements making up the surface renewal zone do not causethe molten fluoropolymer to fill up this zone, i.e. vapor space isavailable and in contact with the small streams of molten fluoropolymerbeing created and recreated by the multiple ZME, TME and SME elements.The surface renewal zone includes a gas injection port positioned nearthe beginning of the zone to feed gas into the zone, if used in theparticular melt processing of the Example. The residence time of themolten fluoropolymer in the surface renewal zone is 35 seconds.

The next 5 L/D contains the vacuum extraction section (devolatilizationzone). The devolatilization zone includes melt forwarding elements thatprovide for free volume, so that the molten polymer is exposed tosubatmospheric pressure so that reactive and corrosive gases do notescape into the atmosphere. The vacuum used in the devolatilization zonein the Examples is 13.7 psia (95 kPa).

Undercut bushings (SK) are an effective way to provide the forwardingelements in the devolatilization zone in the Examples. The final 2 L/Dare used to provide a vacuum seal and pump the molten polymer into thesingle-screw extruder. The vacuum applied to the devolatization zonecommunicates with a cylindrical chamber between the vacuum source andthe devolatilization zone, and the vacuum is applied through a tared 50mesh screen positioned across this chamber. The volatilized oligomercondenses on this screen. Removal of the screen and weighing of thecondensed oligomer while on the screen reveals the amount of oligomerremoved. Comparison of this weight of oligomer with the weight offluoropolymer melt processed in the extruder during the time of oligomercollection on the screen reveals the weight proportion of oligomer thatwas formerly present in the fluoropolymer prior to oligomer removal.Without the presence of the screen or other collection device, theeffect of oligomer removal as part of the melt processing operationcould not be detected.

The waxy solid recovered from the screen is heated in a gaschromatograph (GC) to 250° C. and the resulting gas stream is analyzed.The GC peaks indicate a wide distribution of perfluorinated carboncompounds between C₁₁F₂₄ and C₂₀F₄₂. Some residual material remainsindicating that even higher boiling components are present. Thisanalysis indicates that the waxy solid is a mixture of fluorocarbonoligomers.

The twin-screw extruder empties into a single-screw melt pump, which isdesigned to generate pressure at low shear rates for filtration andpellet formation. The extruded melt is melt cut through a die with 250die holes (2.5 mm). The pellets are cooled by a stream of water.

Both the twin-screw extruder and the single-screw extruder are operatedwith barrel set-point temperatures of 300° C. except for the die, whichis set at 350° C.

Example 1

A compacted flake of a copolymer of tetrafluoroethylene (TFE), with 12.0to 12.3 weight percent hexafluoropropylene (HFP), i.e. HFPI of 3.8, and1.1 to 1.3 weight percent perfluoro(ethyl vinyl ether) (PEVE) commonlyknown as FEP, polymerized with ammonium persulfate (APS) initiator, isused as the feed material. The polymer has an initial melt flow rate(MFR) of 31.9 to 32.5 and is free of alkali and alkaline earth metalarising from ingredients added in polymerization or afterward. Thevacuum system is opened up after operation for 60 minutes withoutinjecting any gas into the surface renewal zone, and a few smallparticles of waxy material are observed on the screen in the vacuumsystem. The weight ratio of waxy material to polymer processed throughthe extruder is 2 ppm. This is determined by subtracting the weight ofthe screen before the run from the weight of the screen plus oligomerafter the run to determine the weight of oligomer collected andcomparing this oligomer weight with the weight of fluoropolymer meltprocessed during the time of the melt processing run.

Example 2

Melt processing similar to Example 1 and using the same fluoropolymer asin Example 1, is conducted except the process is operated for 10 minuteswithout injecting any gas. Nitrogen is injected for 40 minutes at a9,500 ppm weight ratio of nitrogen to fluoropolymer. The screen isremoved. The screen is covered with a waxy solid similar to thatanalyzed in Example 1, but in a much greater amount. The weight ratio ofwaxy material (oligomer) to fluoropolymer processed through the extruderduring the 40 min. run with nitrogen injection is 50 ppm (net amountafter deducting the amount of oligomer collected during the 10 min ofoperation without any nitrogen injection)

Example 3

Melt processing similar to Example 1 and using the same fluoropolymer asin Example 1 is conducted except the process is operated for 5 minuteswithout injecting any gas. A fluorination agent consisting of 10 mole %F₂ in N₂ is then injected into the extruder for 15 minutes at 1300 ppmby weight fluorine. Nitrogen by itself is then injected for 20 minutesat a 9,500 ppm weight ratio of nitrogen to polymer. The run is continuedfor another 5 min without any gas injection to facilitate evacuation offluorine from the molten fluoropolymer. The screen is removed. Thescreen has a thick layer of a waxy solid similar to that obtained inExample 2 but in an even much greater amount. The weight ratio of waxymaterial to fluoropolymer polymer processed through the extruder duringthis melt processing run is greater than 500 ppm (net amount aftersubtracting the amount of oligomer collected during the 30 minutes ofthe run during which no fluorine is injected into the extruder). InExamples 2 and 3, the runs are shut down before reaching 60 min run timeto avoid blockage (plugging) of the vacuum system by the oligomercollecting on the screen. Example 3 was allowed to run even though theoligomer buildup exceeded that of Example 2, until the vacuum could nolonger be drawn on the devolatilization zone.

The amount of oligomer removed in Examples 2 and 3 represent thereduction in oligomer content from the starting fluoropolymer, i.e. theas-polymerized fluoropolymer. From these Examples, it is seen that thatthe minimum removal of 25 ppm oligomer is easily achieved, and whenfluorine used as the gas, removal of at least 200 ppm of oligomer isalso easily achieved. The small amount of oligomer removed when no gasis used as in Example 1 represents a greater amount than would beremoved by conventional extrusion when a devolatilization zone isincluded in the extruder.

In the following Examples, the fluoropolymer itself is tested foroligomer content by the reduction in particulates when subjected to theSmoke Detector Test. The test functions by capturing the voltage signalfrom the ionization chamber of a smoke detector and monitoring it asparticulates are emitted from the fluoropolymer being tested at elevatedtemperatures flow through the ionization chamber. As the oligomerparticles pass through the ionization chamber, the voltage signalchanges, and the magnitude of the change is proportional to the amountof particulates passing through the chamber. By monitoring this voltagechanges, the difference between oligomer content in the resins can bediscerned.

In greater detail, the Smoke Detector Test is conducted as follows: Asample of fluoropolymer is heated in an air-supplied furnace at aspecified temperature. The particulates emitted by the sample areconcentrated (without condensing) by a funnel acting as a hood over theheated sample and flow with the air into a 20 L vessel containing thesmoke detector. The purpose of the 20 L vessel is to provide a hold-upvolume so that the ionization chamber of the smoke detector hassufficient time to detect materials passing through it. Changes in theoutput voltage of the smoke detector are recorded using a simple stripchart recorder. The gases pass out of the 20 L vessel through a watertrap to collect any particulates emitted for subsequent particle sizeanalysis. The particle size analysis shows a strong correlation with thevoltage change, verifying that the smoke detector is a good particulatedetector. Next, the gas stream passes through a flow meter, then tohouse vacuum. Control of the gas flow in the system is desirable sincethe voltage output of the smoke detector is proportional to theconcentration of particulates passing through the ionization chamber.

For the furnace, a Thermolyne Type 6000 ashing furnace is used. Air issupplied to the furnace at 37 slpm. A 5″ (12.7 cm) diameter stainlesssteel funnel welded to ⅜″ (0.95 cm) ID stainless steel tubing is used totransfer the gas from the oven to the 20 L container, a Qorkpak® pailwith a gasketed lid and pouring spout. The stainless steel tubingpenetrates the bucket through a number four stopper in the pouring spoutat the top of the container. The outlet gases and smoke detector voltageleads pass out of the bucket through a 2-hole number 12 stopper pressedinto a 2 in (5.1 cm) hole cut near the bottom of the bucket. Thislocation is selected to insure efficient air flow through the ionizationchamber of the smoke detector.

The smoke detector used is a Kidde®-brand ionization smoke alarm, modelnumber 0916. The smoke detector contains a 0.9μ Curie Americium 241source (half life of 432 years) and a Motorola MC145017P ionizationsmoke detector integrated circuit chip. The detector used is less thantwo years old. The voltage leads are soldered to positions 14, 15, and16 on the chip and power is supplied via a standard 9 V battery. Exposedto air, the voltage output of this circuitry is 4.9-5.0 V. The smokedetector sits in the middle of the 20 L container on a tripod coveredwith a non-conductive filled PTFE gasket.

The strip chart recorder used for this experiment is a Cole-Parmer 100mm 0-5 V model with a chart speed of 1 cm/min. The water vacuum trap inthis set-up is a 250 mL graduated cylinder filled with 100±0.5 gHPLC-grade water from J. T. Baker and is connected to both the 20 Lcontainer and the flow meter with latex tubing. The flow meter is aGilmont size 13 flow meter with a range of 200-14,000 mL/min. Air flowthrough the system, as determined via this flow meter, is 6 L/min and isadjusted directly via the connection to house vacuum.

Fluoropolymer samples tested in this Test are 25 mil (0.64 mm) filmsthat are hot-pressed (5 min at 350° C., then 5 min under cold pressure).Squares of these films weighing 1.00±0.05 g are held at oventemperatures ranging from 200° C. and 350° C. for 60 min while changesin the voltage output of the smoke detector are recorded on the stripchart. These temperatures simulate high service temperatures, 200° C.being the maximum service temperature for FEP, and melt processingtemperature, respectively. Clean water is placed in the vacuum trap foreach experiment and the gases are bubbled through it for 60 min, thesample is removed from the oven, and gases are bubbled through for anadditional 5 min, for a total of 65 min gas collection time. The vacuumtrap is then removed and the system is allowed to purge for anadditional 5 min to remove any residual material—the total system purgetime is therefore 10 min. The heating of the film samples is conductedin small, 5″ (12.7 cm) diameter aluminum pie pans that have beenpreviously heated to 385° C. for 10 min to remove residual manufacturingoils.

For each sample tested, the maximum voltage change is reported as thedifference between the baseline (empty) value and the lowest voltagerecorded on the strip chart during the entire run. This correlates wellwith the integral of the area between the baseline voltage and thevoltage change curve.

Example 4

Film samples of the fluoropolymer obtained from the melt processing ofExamples 1-3 are subjected to the Smoke Detector Test, at 200° C., whichis the upper continuous use temperature for this fluoropolymer, with thefollowing results:

Fluoropolymer Treatment Voltage Change - volts no gas 0.6 nitrogen 0.45fluorine no changeThe voltage change when nitrogen is used is 75% of the voltage changewhen no gas is used, while the voltage change when fluorine is used iseven less than 25% of the voltage change when no gas is used.

Example 5

Film samples of the fluoropolymer obtained from the melt processing ofExamples 1-3 are subjected to the Smoke Detector Test, at 350° C. withthe following results:

Fluoropolymer Treatment Voltage Change - volts no gas 2.4 nitrogen 1.9fluorine 1.7The greater voltage change when the Test is carried out at 350° C. ascompared to 200° C. indicates the relatively large amount of oligomerremaining in the fluoropolymer, i.e. having a significant vapor pressureonly above 200° C.A commercially obtained sample of FEP from another manufacturer, labeledas NP-101, gives a voltage change of 2.5 volts.

1. Melt-processible fluoropolymer having at least about 25 ppm lessoligomer than said fluoropolymer as-polymerized, said fluoropolymeras-polymerized being free of alkali and alkaline earth metal, saidoligomer having the formula C_(x)F_(y), wherein x is at least 11 and yis at least
 24. 2. The melt-processible fluoropolymer of claim 1 havingat least about 200 ppm less oligomer than said fluoropolymeras-polymerized.
 3. The melt-processible fluoropolymer of claim 1 whereinsaid oligomer is a mixture of oligomers having the formula betweenC₁₁F₂₄ and C₂₀F₄₂ and possibly containing even higher boiling oligomers.4. A process comprising melt fabricating melt-processible fluoropolymerhaving at least about 25 ppm less oligomer than said fluoropolymeras-polymerized, after polymerizing said fluoropolymer such that it isfree of alkali and alkaline earth metal, said oligomer having theformula C_(x)F_(y), wherein x is at least 11 and y is at least
 24. 5.Melt-processible fluoropolymer which upon being subjected to the SmokeDetector Test at the upper continuous use temperature for saidfluoropolymer exhibits a voltage change of no more than about 25% of thevoltage change for said fluoropolymer as polymerized, said fluoropolymeras-polymerized being free of alkali and alkaline earth metal.